1. Field of the Invention
The present invention relates to a thermal head which is mounted on a thermal printer and is energized and heated in accordance with printing information to perform a desired printing.
2. Description of the Related Art
In general, a thermal head mounted on a thermal printer is used in contact with a recording medium such as an ink ribbon or a heat-sensitive paper. In such a thermal head, a plurality of heater elements are arranged linearly on a substrate, and some of the heater elements are selectively and sequentially energized and heated so that the thermal head performs printing by coloring a heat-sensitive recording paper in a thermal printer, and by partially fusing ink of an ink ribbon to transfer a plain paper in a thermal transfer printer.
FIG. 3 illustrates one example of the conventional thermal head of this type. Referring to FIG. 3, a plane glaze layer 2 composed of a heat-resistant glass having low thermal conductivity and functioning as a heat insulating layer is formed on the upper surface of an insulating substrate 1 composed of alumina and the like. A protruding portion 2a of the glaze layer 2 having a height of about 5-10 .mu.m and an approximately trapezoidal sectional configuration is formed integrally on the top surface within an area that will form heating portion 3A of the heater elements 3. On the top surface of the glaze layer 2, a plurality of heater elements 3 composed of heat resistant materials such as Ta.sub.2 N, Ta--SiO.sub.2 or the like are formed in line by being totally laminated with vapor deposition or sputtering and then by being subjected to etching of a photolithographic technique. On the top surfaces of both ends of the heater elements 3, a common electrode 4a and an individual electrode 4b are formed, respectively, for energizing each of the heater elements 3. Each of the electrodes 4a and 4b are composed of soft metal having good conductivity such as Al, Cu and A, and formed into desired shapes by being totally laminated with vapor deposition or sputtering and then by being subjected to etching of a photolithographic technique.
Each of the heater elements 3 are formed individually between the common electrode 4a and the individual electrode 4b so that they expose heating portions 3A thereof each corresponding to one dot which is a minimum printing unit. The heating portion 3A of the heater elements 3 is heated through the application of a voltage between the electrodes 4a and 4b.
A protective layer 5 having a thickness of about 7-10 .mu.m is laminated on the upper surfaces of the insulating substrate 1, the glaze layer 2, the heater elements 3 and each of the electrodes 4a and 4b so as to cover the entire surfaces except terminals of each of the electrodes 4a and 4b.
In a thermal transfer printer (not shown) using the conventional thermal head as described above, the individual electrode 4b which is connected to the heater elements 3 selected on the basis of desired printing signals is energized under a condition that the thermal head is pressed into contact with a desired recording medium (not shown) such as a paper carried to the front of a platen through an ink ribbon so as to heat up the selected heater elements 3. By this, the ink of the ink ribbon (not shown) which abuts against the heated heater elements 3 is fused to be transferred to the recording medium so as to perform desired printing of characters and figures thereon.
In the conventional thermal head as described above, desired heater elements 3 are arranged on the protruding portion 2a of the glaze layer 2 to form the heating portion 3A so that abutting property of the thermal transfer printer (not shown) against the platen is enhanced and printing quality is improved.
In recent years, in a thermal transfer printer, efforts have been made to obtain faster printing speed and higher definition by the thermal head and to improve printing quality, and various measures have been taken against the thermal head and the ink ribbon.
As a specific example of obtaining faster printing speed, higher definition and improved printing quality by the thermal head, a thermal transfer printer converting materials of the conventional ink ribbon from wax of a type stripped during low-temperature condition to resins of a type stripped during high-temperature condition has been proposed. With this proposal, the thermal head is, as shown in FIG. 4, made into a so-called real edge type in which a heating portion 3A is provided on the protruding portion 2a of the glaze layer 2 formed by offsetting on the insulating substrate 1 which is a rear end portion with respect to the travel direction of the thermal head, and a distance between the heating portion 3A and the edge portion which contributes stripping of the ink ribbon is short. The thermal head of this type is so constructed as to perform so-called stripping during high-temperature condition in which the ink ribbon is stripped from the recording medium before the ink heated and fused by the heating portion 3A is cooled to be solidified.
However, according to a thermal head of a real edge type in which the protruding portion 2a of the glaze layer 2 on which the heating portion 3A is formed, the space for the common electrode is very small. Thus, it has provided poor yield and led to an increase in cost.
In addition, for the thermal transfer printer in recent years, further speed-up of the printing speed, higher definition and higher printing quality by the thermal head have been required. For example, the printing speed of the thermal head required is 100-150 cps, and the resolution is 360-400 dpi and further moving to 600 dpi. The printing speed is becoming faster, and a smaller size of a dot which is a heat unit approximately equivalent to an area of one heating portion 3A is required. To satisfy this requirement, a resin ink ribbon having higher thermal transfer sensitivity has been proposed.
That is, a resin ink ribbon of a type striped during high-temperature condition, so-called thin film ribbon has been proposed in which the thickness of an ink layer and a base film layer of the ink ribbon are reduced so as to substantially reduce the thickness of the entire ink ribbon from 10 .mu.m to 5 .mu.m. In a thin film ribbon of this type, as the thickness of the ink ribbon is reduced, a thermal transmittal speed of the ink ribbon is increased, and thermal capacity is remarkably lowered. The thin film ribbon is a very sensitive ribbon having high heat resistivity and heat transfer properties such that the ribbon is easy to heat and easy to cool.
The ink ribbon formed to have high sensitivity is, when used for printing, heated to a high temperature by the heating portion 3A and immediately thereafter, pressed into contact with a protective layer 5 outside of the low-temperature common electrode 4a, whereby heat of the ink ribbon is lost quickly. However, unless the ink ribbon is stripped from the recording medium before the ink heated and fused by the heating portion 3A is cooled to be solidified, the ink ribbon is bonded to the recording medium, thereby not only making it impossible to take up the ink ribbon but also deteriorating separating property and fixing property of the ink. Thus, excellent printing quality at an environmental temperature of, for example, 3.degree.-5.degree. C. can not be obtained.
It may be considered that a distance between the heating portion 3A and the edge portion where the ink ribbon is stripped from the recording medium is shortened, and the width of the common electrode 4a is shortened to strip the ink ribbon from the recording medium before the ink is cooled, so that deterioration of separating property and fixing property of the ink can be prevented. However, by shortening the width of the common electrode 4a, voltage drop (common drop) due to the increase of the value of resistance of the common electrode 4a occur, unevenness of the printing density is generated to deteriorate the printing quality and a life of the printing is reduced.
Furthermore, when the ink is in a fused condition, the slip between the ink ribbon and the recording medium tends to occur. When the slip occurs, relative speed is generated between the ink ribbon and the recording medium to cause unstable traveling of the ink ribbon due to oblique traveling thereof, and unsuitably printing on the recording medium.